Harriet Ross was Harriet Tubman's mother.

Harriet Tubman was the fifth of nine children born to Rit Green Ross and Ben Ross. All of Harriet's siblings were enslaved by Edward Brodess, although at least five or six of the Ross children, including Harriet, were born on Anthony Thompson's plantation in the Peter's Neck region of Dorchester County, near the Black Water River at the end of Harrisville Road south of present day Madison. Thompson was the owner of Ben Ross and step-father to Edward Brodess. 

Brodess moved Harriet, her siblings, and her mother Rit, to his own farm in Bucktown, 10 miles to the east of Thompson's farm, sometime after 1823. He forced them to leave behind Ben and the extended familial and community relationships they had forged in the small black community near Thompson's plantation. (1)

Sources 

1. http://www.harriettubmanbiography.com/id14.html
2. Larson, Kate C. Bound for the Promised Land: Harriet Tubman, Portrait of an American Hero. New York: Ballantine, 2004. Print.